Is there a way to introduce a random size for children particles?
I have found what appears to be a solution, however it might be a bug/unexpected (or unsupported) behavior. What you do is you pretend you are going to use the Simple mode, set the random size, and then switch back to Interpolated mode. Some reason the random size data is "remembered" and now the particles are random sizes.



This appears to render correctly, and changing other particle settings doesn't appear to affect it at all.